1. Field of the Invention
The present invention relates generally to a communication system using a multiple access scheme. More particularly, the present invention relates to a transmission/reception method and apparatus for selecting an optimal multiplexing scheme according to data transmission conditions and transmitting data using the selected multiplexing scheme to improve reception performance of data transmitted in a system that transmits/receives data using an Orthogonal Frequency Division Multiplexing (OFDM)-based multiple access scheme.
2. Description of the Related Art
Recently, in the field of wireless communication systems, active research is being conducted on Orthogonal Frequency Division Multiplexing (OFDM), which is useful for high-speed data transmission over wireless channels. OFDM, a scheme for transmitting data using multiple carriers, is a kind of Multi-Carrier Modulation that converts a serial input symbol stream into parallel symbols and modulates each of the symbols with a plurality of orthogonal sub-carriers, such as, sub-carrier channels, before transmission.
FIG. 1 is a block diagram illustrating a structure of a transmitter for a general OFDM system.
Referring to FIG. 1, the transmitter for the general OFDM system includes a channel encoder 101, modulator 102, serial-to-parallel (S/P) converter 103, Inverse Fast Fourier Transform (IFFT) unit 104, parallel-to-serial (P/S) converter 105, and cyclic prefix (CP) inserter 106.
The channel encoder 101 performs channel encoding on an input information bit stream. Generally, a convolutional encoder, turbo encoder, or low density parity check (LDPC) encoder is used as the channel encoder 101.
The modulator 102 performs quaternary phase shift keying (QPSK), 8 phase shifting key (PSK), or 16 quadrature amplitude modulation (QAM) on the output of the channel encoder 101. Although not illustrated in FIG. 1, it would be obvious to those skilled in the art that a rate matching unit for performing repetition and puncturing functions can be added between the elements 101 and 102.
The S/P converter 103 has a function for converting the signal output from the modulator 102 into a parallel signal. The IFFT unit 104 performs IFFT calculation on the output of the S/P converter 103.
The P/S converter 105 converts the output of the IFFT unit 104 back into a serial signal. The CP inserter 106 has a function for attaching a cyclic prefix (CP) to the output signal of the P/S converter 105.
There is a modified OFDM multiplexing scheme in which a transmitter performs Hadamard transform on modulation symbols in a frequency domain before transmission. This scheme is generally called Multi-Carrier Code Domain Multiplexing (MC-CDM) or Orthogonal Frequency Code Domain Multiplexing (OFCDM).
FIG. 2 is a block diagram illustrating a structure of a general Unitary Precoded OFDM transmitter.
Referring to FIG. 2, the general Unitary Precoded OFDM transmitter includes a channel encoder 201, modulator 202, unitary precoder 203, S/P converter 204, Inverse Fast Fourier Transform (IFFT) unit 205, P/S converter 206 and CP inserter 207.
The channel encoder 201 performs channel encoding on an input information bit stream. Generally, a convolutional encoder, turbo encoder, or LDPC encoder is used as the channel encoder 201.
The modulator 202 performs QPSK, 8PSK, or 16QAM modulation on the output of the channel encoder 201. Although not illustrated in FIG. 2, it would be obvious to those skilled in the art that a rate matching unit for performing repetition and puncturing functions can be added between the elements 201 and 202.
The unitary precoder 203 is a common unitary precoder, and various examples of unitary precoding will be described later with reference to FIGS. 3A to 3C.
The S/P converter 204 has a function of converting the output of the modulator 202 into a parallel signal. The IFFT unit 205 performs IFFT calculation on the output of the S/P converter 204.
The P/S converter 206 converts the output of the IFFT unit 205 back into a serial signal. The CP inserter 207 has a function of attaching a CP to the output signal of the P/S converter 206.
FIGS. 3A to 3C are diagrams illustrating multiple examples of the unitary precoder of FIG. 2. FIG. 3A is a diagram illustrating a unitary precoder in which Hadamard transform is used.
Referring to FIG. 3A, the unitary precoder includes a symbol demultiplexer (DEMUX) 311, a Walsh function covering unit 312, and a Walsh summer 313.
The symbol DEMUX 311 converts the serial signal output from the modulator 202 of FIG. 2 into a parallel signal. The Walsh function covering unit 312 performs a process of Walsh-covering or spreading each of modulation symbols output from the symbol DEMUX 311 by a Walsh code with a predetermined length. The Walsh summer 313 performs a process of summing up the outputs (that is, outputs spread by each Walsh function) of the Walsh function covering unit 312.
FIG. 3B is a diagram illustrating a unitary precoder in which Fast Fourier Transform (FFT) is used.
Referring to FIG. 3B, the unitary precoder includes a symbol DEMUX 321, an FFT unit 322, and a P/S converter 323.
The symbol DEMUX 321 converts the serial signal output from the modulator 202 of FIG. 2 into a parallel signal. The FFT unit 322 performs FFT transform on the output of the symbol DEMUX 321. The P/S converter 323 converts the parallel signal output from the FFT unit 322 into a serial signal.
FIG. 3C is a diagram illustrating a unitary precoder in which Fast Frequency Hopping (FFH) is used.
Referring to FIG. 3C, the unitary precoder includes a symbol DEMUX 331, an FFH linear processor 332, and a P/S converter 333.
The symbol DEMUX 331 converts the serial signal output from the modulator 202 of FIG. 2 into a parallel signal. The FFH linear processor 332 performs FFH liner transform on the output of the symbol DEMUX 331. The FFH is a technology for mapping different sub-carriers every OFDM sample. The P/S converter 333 converts the parallel signal output from the FFH linear processor 332 into a serial signal.
Of the foregoing conventional Unitary Precoded OFDM multiplexing schemes, the Hadamard precoded OFDM scheme of FIG. 3A will be referred to as an Orthogonal Frequency Code Division Multiplexing (OFCDM) scheme, and the FFT precoded OFDM of FIG. 3B will be referred to as an FFT Spread OFDM (FFT-S-OFDM) scheme. In addition, the FFH precoded OFDM scheme of FIG. 3C will be referred to as a Fast Frequency Hopping-OFDM (FFH-OFDM) scheme.
FIG. 4 is a diagram of a brief description of a concept of the general FFH introduced in FIG. 3A.
Referring to FIG. 4, a description will be made of a FFH technique by comparing the existing frequency hopping technique with the FFH technique for one OFDM symbol time for an FFT size of M=4.
In FIG. 4, reference numeral 401 denotes a multi-carrier modulation apparatus for performing the existing per-symbol frequency hopping technique for a 4-OFDM sample time. Reference numerals 405 to 408 denote FFH multi-carrier modulation apparatuses.
In the existing frequency hopping technique shown in the left of FIG. 4, input data is identical for a 4-OFDM sample time and output signals are output one by one every sample time. Because the existing frequency hopping technique maps fixed sub-carriers in a one-OFDM symbol time, the blocks denoted by reference numerals 401 to 404 are identical for a 4-OFDM sample time.
However, in the FFH technique shown in the right of FIG. 4, mapping between sub-channel data and actual sub-carriers is changed by an M:M switch every sample time. Sub-carriers to which a first sub-channel denoted by reference numeral 405 is mapped are mapped in order of [1 4 2 3], sub-carriers to which a second sub-channel denoted by reference numeral 406 is mapped are mapped in order of [4 3 1 2], sub-carriers to which a third sub-channel denoted by reference numeral 407 is mapped are mapped in order of [2 1 3 4], and sub-carriers to which a fourth sub-channel denoted by reference numeral 408 is mapped are mapped in order of [3 2 4 1]. This mapping pattern is called a hopping pattern for each sub-channel.
As described above, compared with the Unitary Precoded OFDM technology, the OFDM technology shows higher performance at a low code rate. However, compared with the OFDM technology, the Unitary Precoded OFDM technology exhibits higher performance at a very high code rate, such as, a 4/5 code rate. Despite these characteristics, the common mobile communication system undesirably uses one of the OFDM and Unitary Precoded OFDM technologies for the packet data transport channels.
Accordingly, there is a need for an improved adaptive data multiplexing method in an OFDMA system and transmission/reception apparatus that uses OFDM and unitary precoded OFDM technologies.